1. Field of the Invention
The present invention relates to a code division multiple access (CDMA) system including a spread spectrum communication system, and particularly to a code division multiple access system for increasing the speed of synchronous acquisition processing and also enhancing high secrecy.
2. Description of the Related Art
According to the spread spectrum communication system, a spread code (ordinarily, pseudo noise code (hereinafter referred to as "PN code")) is generally modulated by data at a transmission side to spread the frequency spectrum, and a received code and the PN code are subjected to correlation processing at a reception side. If both the codes of the transmission side and the reception side are coincident with each other or by synchronizing both the codes with the self-correlation wave of large amplitude appearing in the neighborhood of the codes, the data can be decoded without suffering the effect of multipass and narrow-band noise.
According to a direct spread spectrum communication system which is one type of spread spectrum communication system, the data are multiplied by a PN code having a chip rate which is remarkably higher than the data rate to spread the spectrum. In this system, a simultaneous multiplex communication in the same frequency band can be performed by discriminating different PN codes or PN codes which are different in absolute phase. This system is also called as CDMA (Code Division Multiple Access) system, and a number of studies have been made to enable practical use of this system to a radio-communication such as a mobile communication or a radio LAN.
FIG. 20A shows the construction of a general transmission portion of the CDMA system, and FIG. 20B shows the construction of a general receiving portion of the CDMA system.
At the transmission unit, the data are first input to an encoder 126. In the encoder 126, an error correction capability such as convolution encoding of the input data, etc. are added, and a secrecy function such as interleave, etc. are added. The data output from the encoder are input to a spread modulator 127, and multiplied by the PN code (whose chip rate is remarkable higher than the bit rate of the data) generated in a PN code generator 128 by a multiplier, whereby the data are subjected to spectrum spread processing.
The PN code generator 128 is driven by a reference clock from a reference signal generator 129. The spectrum-spread data are frequency-converted to an RF band in a radio modulator 130, amplified to a desired level in an amplifier 131 and then radiated from an antenna 132. The data may be subjected to phase modulation such as BPSK or the like in the radio modulator 130. At the reception unit, as shown in FIG. 20B, the signal received from an antenna 140 is amplified by an amplifier 139, down-converted to an IF band in a radio demodulator 138, and then input to a spread demodulator 134. The reception unit is provided with a PN code generator 137 for generating the same PN code as the PN code generator 128 at the transmission unit, and the PN code generated in the PN code generator 137 and the down-converted reception signal are multiplied by a multiplier. Here, when the PN code multiplied by the reception signal and the PN code generated in the PN code generator 137 are not synchronized with each other, the integration value (correlation value) of the multiplication result of both the PN codes exhibits an average noise level, however, when the synchronization therebetween is established, the correlation value has a peak.
Accordingly, a phase controller 136 controls the phase of the PN code of the PN code generator 137 so that the peak is detected (with [?] pulse detector 135) and the peak can be continuously detected. When the synchronization is established as described above, the despread of the reception signal succeeds, so that the reception signal of the base band from which the PN code is removed can be decoded. In a decoder 133, the scramble of the data is released by deinterleaving the data, error correction is applied by a viterbi decoding, and then the decoded reception data are picked up. In this process, a narrow-band noise and a multipass signal which are added in a transmission path between the transmission unit and the reception unit are subjected to spectrum spreading, and reduced to such a level that they have no effect on the reception of the desired wave.
When the spread spectrum communication system is applied to a mobile communication system (cellular system) for example, in many cases another PN code which is different from the spread code having the above basic construction is used in combination with the spread code. The basic system is constructed by plural personal stations for every master station. However, in the mobile communication system, each personal station (mobile station in the mobile communication) is required to identify a master (base) station to which the personal station communicates because there are plural base stations as a master station in the mobile communication system. Particularly in the mobile communication (cellular system), the communication is carried out while a mobile station moves between base stations each constituting an area which is called as a cell, and thus the identification of the base station is indispensable to support a technique inherent to the CDMA which is called as "soft hand-off" occurring in the neighborhood of the cell boundary.
Therefore, individual PN codes or PN codes which are different in absolute phase are allocated to the respective base stations in addition to the PN code for spreading the data, and then multiplied by the data after the spectrum spread processing as if the transmission data are provided with the color of each base station, whereby the mobile station can identify each base station serving as a transmission side of the reception data.
FIGS. 21A and 21B show the basic construction of a transmission unit of a base station and a reception unit of a mobile station which are used in the mobile communication system. In a multiplier 141, the data which is subjected to the spectrum spreading in the spread modulator 127 are multiplied by a base-station identifying PN code which is generated in another PN code generator 142, and then input to the radio modulator 130. The other operation is the same as shown in FIG. 20.
In the reception unit of the mobile station, the base-station identifying PN code which is the same as the transmission unit of the base station is generated in a PN code generator 144, and in the multiplier, the PN code thus generated is multiplied by the reception data which is down-converted in the radio demodulator 138. When the synchronization is established between the PN code of the transmission unit of the base station contained in the reception data and the base-station identifying PN code which is multiplied in the reception unit of the mobile station, the base station identifying PN code is removed from the reception data. The reception signal from which the base-station identifying PN code is removed is despread in the spread demodulator 134, and decoded in the decoder 133 to restore the data.
In the spread spectrum communication system, it is a key point of the data restoration whether the synchronization can be established with the spread code contained in the reception data at the reception side. In other words, this means the secrecy to an irregular receiver. Therefore, various studies have been made to enhance the secrecy and shorten the initial synchronizing time at the reception side on the spread spectrum communication system.
A technique as disclosed in Japanese Laid-open Patent Application No. Sho-63-127634 is one of the above studies. According to this study, in addition to a technique for "adding a synchronous signal to the head of a data frame" which has been hitherto considered, a PN code which is exclusively used for synchronization (i.e., synchronization-only PN code) and is synchronized with the spread code of data is transmitted from a master station completely independently of the data and also continuously, whereby a personal station is beforehand synchronized with the synchronization-only PN code to shorten the synchronous acquisition time with the spread code in the despread processing of the data.
FIGS. 14 and 15 show a conventional technique, and FIG. 16 shows an example of transmission data of a master station. Now, the operation when the data are transmitted from a master station to a personal station as shown in the figures by using the conventional technique will be described.
First, a PN code II generator 96 and an PN code I generator 97 generate different PN codes in synchronism with a clock of a clock generator 98. However, the relative phase of the two PN codes is predetermined. The PN code generated in the PN code II generator 96 is amplified in power by an amplifier 99, and transmitted at all times. The PN code generated in the PN code I generator 97 is modulated by a modulator 100 only when there is transmission data, and then amplified and transmitted by the amplifier 99.
Further, in the personal station, the received PN code II is correlated with the output of a PN code II generator 104 by a correlator 109, and only the PN code II for synchronization is selected and demodulated in a demodulator 110. A one-period search circuit 107 varies the oscillation frequency of an oscillator of the clock generator 105 to vary the phase of the PN code II. In this case, the search is performed over one period of the PN code II. Therefore, the time required for synchronous acquisition is long, however, once the synchronization is established, the synchronization will have been subsequently continuously established. A PN code I generator 106 operates in synchronism with the clock generator and the PN code II generator 104, and it is coincident with the relative phase which is determined in the master station. The PN code I output from the PN code I generator 106 is correlated with the reception signal by a correlator, however, in this case, the synchronization can be established in a short time by delaying the phase of the PN code I generator 106 by a chip search circuit 112.
Further, there is also known another conventional technique as disclosed in Japanese Laid-open Patent Application No. Hei-05-110538. Unlike the conventional technique described above, this conventional technique pays much attention to the secrecy of communication. FIGS. 17 and 18 show an example of the conventional technique. In FIGS. 17 and 18, reference numeral 115 represents a carrier wave generating circuit, reference numeral 116 represents a PSK modulation circuit, reference numeral 117 represents a spread spectrum mixing unit, and reference numeral 123 represents a PN sequence generating circuit. Reference numeral 123a represents a PN sequence generating circuit for the transmission side, and reference numeral 123b represents a PN sequence generating circuit for the reception side. In the PN sequence generating circuit, reference numeral 121 represents an FIFO (first-in first-out) element, reference numeral 118 represents a spread sequence clock generator, reference numeral 119 represents a frequency divider, and reference numeral 122 represents a PN sequence B generating circuit, which comprises K-staged shift register. Reference numeral 120 represents a DSP (digital signal processor), and PN sequence A data of two periods are stored in a built-in ROM. Reference numeral 124 represents a PSK demodulation circuit.
Next, the operation will be described. In this conventional technique, three modes of a mode 1, a mode 2 and a mode 3 are provided as a communication mode as shown in FIG. 19. Of these modes, in mode 1, the transmission side subjects a spread sequence A with no phase shift to spread modulation by A110 data, and then transmits the spread-modulated spread sequence A. The reception side establishes the synchronization of clocks of the spreading PN sequence during the reception of the signal of the mode 1. In mode 2, the transmission side spread-modulates the spread sequence A with no phase shift by M sequence data as in the same manner as described above. The M sequence data are set to notify it to the reception side that the phase of the PN spread sequence A is just afterwards shifted every period to be spectrum-spread. At this time, the coincidence between the reception data and the M sequence is checked by DSP 120 at the reception side, and if both are coincident with each other, the mode is shifted to the next mode 3. In the mode 3, the transmission side drives the K-staged shift register in the PN sequence B generating circuit 122 for phase shift by a clock obtained by frequency-dividing the clock for the spreading PN sequence A data into 1/(2.sup.k -1) in the frequency-divider 119.
The K-staged shift register in the PN sequence B generating circuit 122 is shifted every one period of the spreading PN sequence A. The state of the K-staged shift register is read out by the DSP 120, and this value is converted to decimal numbers. The converted decimal number is given as a shift amount to the spreading PN sequence A, and serially input into an FIFO element 121. The phase-shifted PN sequence A data are output as an FIFO output from the FIFO element 121 by the clock for the spreading PN sequence A data. The FIFO output is multiplied by the output of the PSK modulation circuit 116 in a mixer at a subsequent stage to perform the spectrum spreading. As described above, the PN sequence for the data spreading is shifted on the basis of another PN sequence having a predetermined period every one period and then transmitted/received. Therefore, even when a third party knows the type of the spreading PN sequence and the clock synchronization is established, the third part can be prevented from continuing to eavesdrop the data.
The conventional spread spectrum communication systems as described above has a problem that the time required for the initial synchronization at the reception side can be shortened while keeping the communication secrecy. The following is the reason for this.
In the first conventional prior art, the initial synchronization at the reception side can be facilitated by continuously transmitting the spread code exclusively used for the synchronization. However, at the same time this conventional technique enables a third party to establish the initial synchronization, and thus the communication secrecy is not guaranteed. In order to enhance the secrecy in this conventional technique, the code length of the synchronizing PN code may be lengthened. However, this method also lengthens the initial synchronization time by a regular receiver, and thus the original object of this technique is lost.
Further, in the second conventional technique, the processing for enhancing the secrecy is complicated, so that it takes a long time until the initial synchronization is established at the reception side, and the construction of an implemented device is complicated.